The present invention relates generally to circuit boards and more particularly to impedance matched circuit boards.
Conventionally, in order to connect a high-frequency coaxial connector for transmitting signals at high frequencies such as a microwave to a microstrip line formed on a circuit board, a circuit structure arranging a ground layer around the microstrip line and at the rear side of the circuit board is used. This is shown in Japanese Utility Model Application Laid-Open (Kokai) No. 5-82111.
FIG. 4 is a view showing a circuit structure, which is formed on a surface of a conventional circuit board, and FIG. 5 is a view showing a circuit structure, which is formed on other surface of the conventional circuit board.
In FIGS. 4 and 5, 801 denotes a first ground layer formed by a metallic film and formed on one face of a circuit board made of a dielectric material, and 802 denotes a second ground layer formed by a metallic film formed on other face of the circuit board. In addition, 803 denotes a first through-hole, or via, penetrating through the circuit board; and circular first relief part 808 and circular second relief part 809, from which the first ground layer 801 and the second ground layer 802 are removed at a predetermined radius, are formed around the first through-hole 803. 804 denotes a second through-hole penetrating through the circuit board, and a plurality of such through-holes 804 are arranged so as to encircle the periphery of the first through-hole 803 in a horseshoe shape. Further, conductive metal plating is applied onto the inner circumferential faces of the first through-hole 803 and the second through-holes 804.
In addition, a soldering part 806 is formed on the first via 803 on one surface of the circuit board and the end of a microstrip line 807 made of a metallic film is connected to the first through-hole 803. Then, a central conductor of the high-frequency coaxial connector (not illustrated) is connected to the first via 803 by the soldering part 806. Further, the external conductor of the high-frequency coaxial connector is connected to the second ground layer 802.
Here, the first ground layer 801 is connected to the second ground layer 802 by the second vias 804, so that the external conductor of the high-frequency coaxial connector, the vias 804, the first ground layer 801, and the second ground layer 802 have the same potentials. Therefore, the central conductor of the high-frequency coaxial connector and the first via 803 and the microstrip line 807, which are connected to this central conductor, are encircled by the external conductor of the high-frequency coaxial connector, the second vias 804, the first ground layer 801, and the second ground layer 802 having the same potentials, so that it is possible to make an electric property of a connecting portion providing an electric connection between the high-frequency coaxial connector and the circuit board stable.
Nevertheless, according to the conventional circuit structure, since both of the first ground layer 801 and the second ground layer 802 are formed around the first via 803 and the second via 804, a third layer must be provided for forming the microstrip line 807.
In addition, a first opening 808 and a second opening 809 formed on the first ground layer 801 and the second ground layer 802, respectively, are positioned inside the second vias 804, and this results in that they have substantially the same shapes and sizes. Therefore, appropriate matching of electric impedance cannot be accomplished.